20% looks amazing, but is illegal. Get it if you want, but be prepared to be told to remove it. 30% is nice as well, but definitely not as striking as 20% (and, of course, it has less UV protective value) and won't get you as much attention from the powers that be. I did 30 on my front windows and 20 on the rear three windows and have had no issues.
FWIW, if you have a dark interior, your dark tint will look darker and garner you more unwanted attention than had you gone with 30 or 35. If you have a dark exterior, the lower degree of contrast between your dark paint and your darker window tint will emphasize the darkness of your tint. If you have black inside and black or very dark blue outside, you should definitely veer closer to what's legal (mainly for your wallet's sake) than toward what's not legal.
Regardless of what illegal tint value you select, it's far more likely to be noticed and measured if you get stopped for something else. If you do get stopped, turn on all your interior lights so you can be seen more easily; it may buy you just enough goodwill so that the cop doesn't concern himself with your tint. Whatever you choose, try to think of the situation from a cop's point of view.
They know that a large quantity of apprehensions for "significant" offenses result from otherwise routine traffic stops. So that means that a large quantity of the folks stopped for a traffic violation are, in the cops' minds, potentially hazardous to their well being, particularly since, unlike in the old days, there is one, not two, cop per cruiser. Anything you can do to reduce their initial tension level when approaching your vehicle will help you.
Not directly relevant, but this may help provide some perspective:
I wish I knew why they stopped doing the two cops per car thing. I presume it's because:
- Splitting them up is a less expensive way to make it look like there are more cops on patrol. Voters likely prefer more police presence/visibility to more actual police force. So what you have is two eyes seeing less detail in a given space, but hundreds of citizens eyes seeing what appears to be more cops on the street. There aren't, there are just more cars; payroll costs far more than cars.
- If a perpetrator is going to bring harm to the officer, he's able to make less of an impact on the overall force if only one cop is harmed, leaving the other, already in route and aware of what s/he's going to encounter.
(I find it hard to believe that the last thing a cop does before exiting his/her vehicle is inform someone that that's what s/he's doing. And that someone knows that if they don't hear back from the cop in 1.5 to 2 minutes -- if that -- that something has gone awry. Indeed, I suspect there's a whole timing protocol for when the first cop actually exists his/her cruiser and when support is notified, and in route and X minutes away from the scene and the time between communications.)